1. Field of the Invention
The present disclosure relates to a liquid crystal display (LCD) panel, and more particularly, to a high quality LCD panel having an even cell gap.
2. Description of the Prior Art
The LCD panel has been adopted as a display device of various electronic products, such as televisions and computer monitors, due to its small size, light weight, and low power consumption.
An LCD panel includes an array substrate, a CF (color filter) substrate facing the array substrate and keeping a cell gap from the array substrate, and liquid crystal molecules filled between the array substrate and the CF substrate. Basing on the direction of an electric field applied to the substrate, LCD panels are divided into a longitudinal electric filed type LCD panel, such as TN (twisted nematic) mode and STN (super twisted nematic) mode, and a latitudinal electric field type LCD panel, such as IPS (in-plane switching) mode that allows wide viewing angles. In either type of LCD panel, the liquid crystal molecules are electrically controlled by the array substrate, and the light transmittance of a backlight is filtered by the CF substrate so as to display colorful images.
In the past, spacers, mostly spherical spacers composed of plastics, have been spread out between the array substrate and the CF substrate for keeping the cell gap. In that case, the diameter of the spherical spacers are a crucial factor to the yield, and distributional accuracy of the spherical spacers is also required. However, it is difficult to precisely control the distributional density of the spherical spacers, and those unevenly distributed spherical spacers diffract or block the backlight. Consequently, the display quality of the LCD panel is reduced.
Please refer to FIG. 11. FIG. 11 is a schematic diagram of a conventional LCD panel 110. As shown in FIG. 11, column spacers 116, which have been adopted recently, are used to replace the spherical spacers. The LCD panel 110 includes an array substrate 112, a CF substrate 114 facing the array substrate 112 and offset by a cell gap from the array substrate 112, and a plurality of column spacers 116 positioned between the array substrate 112 and the CF substrate 114 for keeping the cell gap.
Liquid crystal molecules 120 are implanted into a display region between the array substrate 112 and the CF substrate 114. The array substrate 112 and the CF substrate 114 are compressed with a sealing load to adjust the cell gap, and a sealing gel 126 is applied to the edges of the array substrate 112 and the CF substrate 114. In addition, the CF substrate 114 further includes a black matrix 124 for shielding the boundary region between pixels. Two alignment films 118 are respectively positioned on the surface of the array substrate 112, and the surface of the CF substrate 114 and the column spacers 116. Furthermore, two polarizers 122 are respectively positioned on the surface of the array substrate 112 and the surface of the CF substrate 114 that are not in contact with the liquid crystal molecules 120.
In addition to the method of adjusting the cell gap by applying a sealing load on the array substrate 112 and the CF substrate 114 (referred to as an LC implantation method) as previously mentioned, another method for implanting the liquid crystal molecules 120 (referred to as one drop fill method, ODF method) has been recently developed. According to the ODF method, the column spacers 116 are not applied with a sealing load for adjusting the cell gap between the array substrate 112 and the CF substrate 114. Instead, the cell gap between the array substrate 112 and the CF substrate 114 is adjusted by virtue of controlling the amounts of the liquid crystal molecules 120. It is worthy of note that the LC implantation method can be applied to an LCD panel with spherical spacers or column spacers 116, but the ODF method can only be adopted when the LCD panel is equipped with column spacers 116.
The column spacers 116 are formed, for instance, by forming an epoxy resin layer or a propylene resin layer with an even thickness on the array substrate 112 or the CF substrate 114, and performing a photolithography process to pattern the epoxy resin layer or the propylene resin layer. Therefore, the position of the column spacers 116 is precisely controlled. For example, if a display region is the region that actually displays images, and an aperture region is the region that allows the backlight through, the column spacers 116 are preferably formed in a non-aperture region that overlaps with the black matrix 124. This is because if the column spacers 116 are formed in the aperture region, the aperture ratio of the display region is reduced. In addition, the orientation of the liquid crystal molecules 120 close to the column spacers 116 is disordered, which leads to an uneven brightness problem.
In another aspect, a fixed and even cell gap is critical for improving and enhancing the display quality of the LCD panel 110. An uneven cell gap results in display defects, such as a color defect or a contrast defect. Therefore, for ensuring high display qualities, e.g. high-evenness of display, high contrast ratio, and high response time, a fixed and even cell gap is strictly required.
However, the thermal expansion of the liquid crystal molecules 120 and the column spacers 116 due to heat accumulated in the LCD panel 110, the inner pressure distribution of the liquid crystal molecules 120 in the LCD panel 110 due to gravity, or other factors makes it difficult to maintain a fixed cell gap.
Please refer to FIG. 5(a) and FIG. 5(b). FIG. 5(a) is a schematic diagram illustrating an LCD panel 110 in a vertical position. FIG. 5(b) is a schematic diagram illustrating an inner pressure distribution due to gravity that acts upon the LCD panel 110 shown in FIG. 5(a). The LCD panel 110 includes spherical spacers (not shown), and is sealed with a sealing load. The inner pressure distribution P(y) of the liquid crystal molecules 120 in the LCD panel 110 can be expressed by Equation 1:P(y)=P0+ρ·g·y (−h/2≦y≦h/2)wherein                P0 denotes an average inner pressure of the liquid crystal molecules when sealed;        ρ denotes the specific gravity of the liquid crystal molecules 120;        h denotes the height of the LCD panel 110 in the display region; and        g denotes the acceleration of gravity.        
FIG. 6 is a schematic diagram illustrating a cell gap of the LCD panel shown in FIG. 5(a). In FIG. 6, the y′ axis represents the height (mm) measured from the bottom of the LCD panel 110, the x axis represents the distance (mm) measured from the left end of the LCD panel 110, the CG axis represents the cell gap (μm) of the LCD panel 110, and y′ and y have a relation of y′=−y+h/2. As shown in FIG. 6, the cell gap becomes larger in the lower portion of the LCD panel 110 in which the inner pressure of the liquid crystal molecules 120 is higher, and the cell gap becomes smaller in the upper portion of the LCD panel 110 in which the inner pressure of the liquid crystal molecules 120 is lower.
Recently, large-sized TVs are in demand, and therefore the demand for large-sized LCD panels increases. Normally, an LCD panel, especially a large-sized LCD panel for use in a TV, is in a vertical position when viewed, thus the inner pressure distribution of the liquid crystal molecules due to gravity is uneven. This uneven inner pressure distribution of the liquid crystal molecules leads to a cell gap difference between the upper portion and the lower portion of the LCD panel. In addition, the operation temperature in the LCD panel is generally about 50 to 70 degrees Celsius due to the heat generated by the backlight. In that case, the cell gap difference is significant, resulting in display defects known as gravity mura.
Specifically, gravity mura occurs when the LCD panel is in a vertical position at a high temperature. If the thermal expansion of the liquid crystal molecules exceeds the elastic deformation range when the LCD panel is sealed, the liquid crystal molecules in the cells accumulate in the bottom of the LCD panel due to gravity. This leads to an uneven cell gap.
Please refer to FIG. 9(a) through FIG. 9(c). FIG. 9(a) through FIG. 9(c) are schematic diagrams illustrating how gravity mura occurs. As shown in FIG. 9(a), a sealing load is applied to the array substrate 112 and the CF substrate 114 (i.e. the column spacers 116) at atmospheric temperature, and the column spacers 116 have elastic deformations due to the applied sealing load. When the LCD panel 110 is heated, the liquid crystal molecules 120 expand. This increases the cell gap, and therefore decreases the elastic deformations of the column spacers 116. If the expansion of the column spacers 116 is not as large as the expansion of the cell gap, the column spacers 116 separate from the array substrate 112 or the CF substrate 114 as shown in FIG. 9(b). If the LCD panel 110 is in a vertical position, the liquid crystal molecules 120 flow downwards and accumulate in the bottom of the LCD panel 110 due to gravity. Consequently, gravity mura occurs in the lower portion of the LCD panel 110 as shown in FIG. 9(c).
Please refer to FIG. 7(a) and FIG. 7(b). FIG. 7(a) is a cross-sectional view of a large-sized LCD panel in a vertical position. FIG. 7(b) is a schematic diagram illustrating an inner pressure distribution due to gravity that acts upon the LCD panel shown in FIG. 7(a). Differing from the LCD panel shown in FIG. 5(a), the LCD panel 110 shown in FIG. 7(a) is large-sized, and therefore the relation between the height y and the atmospheric pressure can be expressed as follows:y=yatm(−h/2≦yatm≦h/2)
In the large-sized LCD panel 110, if the liquid crystal molecules accumulate in the lower portion of the LCD panel 110, the inner pressure of the liquid crystal molecules 120 far exceeds the atmospheric pressure when yatm≦y≦h/2. Consequently, the cell gap distribution is as FIG. 8 shows. In FIG. 8, the origin (L=0 mm) and the right end (L=20 mm) of the latitudinal axis respectively represent y=yatm and y=2/h in FIG. 7(a), wherein y=h/2 is the position of the sealing gel 126. In addition, the longitudinal axis in FIG. 8 represents the cell gap variation compared to the original length of the column spacers 116, in which 0.6 t, 0.7 t, and 1.1 t respectively represent conditions in which the CF substrate 114 has a thickness of 0.6 mm, 0.7 mm, and 1.1 mm. It can be seen from FIG. 8 that the thinner the thickness of the CF substrate 114 is, the larger the cell gap variation becomes.
Japanese Patent No. 2002-37325 discloses a method for reducing the cell gap difference when the LCD panel is in a vertical position, so as to reduce display defects. The LCD panel of Japanese Patent No.2002-37325 includes a pair of substrates facing each other, a plurality of column spacers for keeping the cell gap formed on at least one of the substrates, and liquid crystal molecules filled between the substrates. The LCD panel is characterized by keeping the column spacers in the elastically deformed condition at a temperature from 25 to 50 degrees Celsius when the LCD panel is in a vertical position.
However, Japanese Patent No. 2002-37325 only considers the thermal expansion factor that affects the cell gap of the LCD panel, and fails to consider the gravity factor that also affects the cell gap.